"When, in 1671, the director of the Paris Observatory, using an improved telescope, saw Iapetus for the first time, Saturn was no longer the Galileo’s "planet with ears”. The astronomer Christiaan Huygens had already solved the mystery in 1659 when he realized that the "ears” of Saturn were really a system of rings. Iapetus was mysterious from the beginning. The astronomer was able to see it only on the western side of the Saturn. It wasn’t until 1705 that Giovanni Cassini finally saw Iapetus on the eastern side of the great planet using a better telescope because the moon appeared much dimmer on that side. He correctly assumed that Iapetus has a much darker side than the other.

310 years later, the Voyager 2 spacecraft was able to show Iapetus from a closer perspective. It showed that the two contrasting sides make it appear like a Yin/Yang symbol. The leading hemisphere and sides are darker and the trailing hemisphere and poles are almost as bright as the icy world of Europe. With a density of 1.083 grams per cm3 it was clear that the bright regions are made of water ice. But Voyager’s missions let the mystery of the dark material still unsolved. Scientists suggested that it is made of debris from the Saturn’s largest ring, which orbits in the opposite direction of Iapetus. This would explain why the darker Cassini Region is located on the "front” leading side of the moon.

In 2007, the Cassini spacecraft flew even closer to Iapetus (1700 km). The better images showed that Iapetus is indeed receiving reddish dust during its orbit around Saturn. The slow orbital speed around Saturn (79 days) is exposing the dark regions to the sun for long periods of time. The dark material absorbs much more sunlight than the light areas, which reflect sunlight. This causes the ice under and near the dark material to evaporate and redeposit on the bright side making the dark areas even darker and bright areas even brighter.

Cassini mission also added more mystery to the already mysterious world of Iapetus. Its shape is not spherical, but walnut like, which suggests that long time ago Iapetus was spinning very fast. In the middle of the Cassini Region, along the equator, there is a mountain ridge about 1300 km long. Peaks are rising more than 20 km above the plains, making them some of the tallest mountains in the entire Solar System.

After almost 340 years from its discovery, Iapetus continues to refuse to reveal all its secrets. There are still many things to be explained. Why it has this strange shape? Why its orbit is inclined to Saturn’s ring plane? How were the mountains formed? Why are they located in the middle of the Cassini Region?

The beautiful story of discovering and explaining Iapetus and the mysteries that are still waiting to be solved make this target the right choice for further exploration and might help us uncover truths generally applicable to the entire Universe."